Low-head dams have diverse impacts on fish population genetics

Once an essential approach to providing for industrial, recreational and agricultural demands, dams across the country are now being removed in a push to restore waterways to their original configurations.

The removal of large dams—ones higher than 25 feet—is often accompanied by a plan to monitor the affected eco- systems. But the same level of attention hasn’t been given to understanding the ecological impacts of disassembling low- head structures, which account for approximately 48 percent of dams nationwide.

That is until researchers at Eastern Illinois University (EIU) used Illinois Water Resources Center funding to assess the impact of two low-head dams on the genetic diversity and differentiation of fish communities in the Vermilion River. The Ellsworth Park and the Danville structures were slated for removal as early as 2014 but still stand due to budget constraints.

The investigation focused on two fish species with important behavioral differences: longear sunfish, an animal known to live its life within a small area, and the bluntnose minnow, which traverse the river basin in big schools.

“We wanted those two different life histories,” said Robert Colombo, an EIU biologist and the project lead. “One that’s a homebody that wouldn’t be as impacted by the dams and one that moves a lot. We would expect there might be some impacts on bluntnose minnows.”

The researchers isolated specific fragments of the fish’s DNA where mutations tend to occur and compared the genetic pairs from samples taken above and below the dams.

“We looked at microsatellites,” said Shannon Smith, who recently received her master’s from EIU. “Microsatellites are sequences of repeating nucleotide base pairs in a non-coding section of the fish’s genome. They exhibit high variation among individuals in a population, and that gave us a good way to look at any sort of genetic differentiation that was occurring in these species.”

Despite the small range of longear sunfish, Colombo and Smith found no differences in the number of genetic characteristics or the frequency with which those characteristics appear in fish above and below the dams, meaning divided populations are still interbreeding.

“When the discharge is high and the structures are really low, they may be able to traverse up stream,” Smith explained. “All in all, the longear sunfish were largely unaffected by the dams in terms of genetics.”

The bluntnose minnow was a slightly different story.

“We found two distinct genetic populations in the stretch of the river that we studied, and the differentiation point was the Ellsworth Park Dam,” said Smith. “The bluntnose above the dam were genetically distinct from those below that dam and in the other sites we looked at in the Vermilion River.”

This research took place over three years and involved support from the Illinois Department of Natural Resources and U.S. Geological Survey.

“All of these people coming together to determine the best strategy for the project—those are the type of projects that are the most successful because you get the most minds on it,” Colombo said. “I think that’s what makes science best: when you have good collaborators all coming together for a common goal.”

Colombo and Smith hope to continue and expand the project if the dams are in fact removed.

“That would be really neat since we’ve got a few years of pre-removal data to compare it to,” said Smith, who now works for the U.S. Fish and Wildlife Service in South Dakota. “A lot of studies don’t have a before and after big picture of what’s going on.”